The
last universal ancestor (
LUA), also called the
last universal common ancestor (
LUCA), or the
cenancestor, is the most recent
organismIn biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
from which all organisms now living on Earth
descendIn evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
. Thus it is the
most recent common ancestorIn genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
(MRCA) of all current life on Earth. The LUA is estimated to have lived some
3.5 to 3.8 billion years agoThis timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
(sometime in the
PaleoarcheanThe Paleoarchean is a geologic era within the Archaean. It spans the period of time 3600 Ma to 3200 Ma —the period being defined chronometrically and not referenced to a specific level in a rock section on Earth. The oldest ascertained life form is from this period....
eraAn era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in...
).
The
last universal ancestor (
LUA), also called the
last universal common ancestor (
LUCA), or the
cenancestor, is the most recent
organismIn biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
from which all organisms now living on Earth
descendIn evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
. Thus it is the
most recent common ancestorIn genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
(MRCA) of all current life on Earth. The LUA is estimated to have lived some
3.5 to 3.8 billion years agoThis timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
(sometime in the
PaleoarcheanThe Paleoarchean is a geologic era within the Archaean. It spans the period of time 3600 Ma to 3200 Ma —the period being defined chronometrically and not referenced to a specific level in a rock section on Earth. The oldest ascertained life form is from this period....
eraAn era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in...
).
Charles DarwinCharles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
proposed the theory of universal common descent through an
evolutionEvolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary process in his book
On the Origin of Species, saying, "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."
Features
Based on the properties currently shared by all independently living organisms on Earth, it is possible to deduce the defining features of the LUCA.
- The genetic code
The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....
was based on DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
.
- The DNA was composed of four nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s (deoxyadenosineDeoxyadenosine is a deoxyribonucleoside. It is a derivative of the nucleoside adenosine, differing from the latter by the replacement of a hydroxyl group by hydrogen at the 2' position of its ribose sugar moiety. Deoxyadenosine is the DNA nucleoside A, which pairs with deoxythymidine in...
, deoxycytidineDeoxycytidine is a deoxyribonucleoside. It is like cytidine, but with one oxygen atom removed....
, deoxythymidine and deoxyguanosineDeoxyguanosine is composed of the purine nucleoside guanine linked by its N9 nitrogen to the C1 carbon of deoxyribose. It is similar to guanosine, but with one hydroxyl group removed from the 2' position of the ribose sugar . If a phosphate group is attached at the 5' position, it becomes...
), to the exclusion of other possible deoxynucleotides.
- The genetic code was composed of three-nucleotide codons, thus producing 64 different codons. Since only 20 amino acids were used, multiple codons code for the same amino acids.
- The DNA was kept double-stranded by a template-dependent DNA polymerase
A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
.
- The integrity of the DNA was maintained by a group of maintenance enzymes, including DNA topoisomerase, DNA ligase
In molecular biology, DNA ligase is a specific type of enzyme, a ligase, that repairs single-stranded discontinuities in double stranded DNA molecules, in simple words strands that have double-strand break . Purified DNA ligase is used in gene cloning to join DNA molecules together...
and other DNA repairDNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
enzymes. The DNA was also protected by DNA-binding proteins such as histones.
- The genetic code was expressed via RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
intermediates, which were single-stranded.
- RNA was produced by a DNA-dependent RNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
using nucleotides similar to those of DNA with the exception of thymidineThymidine is a chemical compound, more precisely a pyrimidine deoxynucleoside. Deoxythymidine is the DNA nucleoside T, which pairs with deoxyadenosine in double-stranded DNA...
in DNA was replaced by uridineUridine is a molecule that is formed when uracil is attached to a ribose ring via a β-N1-glycosidic bond.If uracil is attached to a deoxyribose ring, it is known as a deoxyuridine....
in RNA.
- The genetic code was expressed into proteins.
- All other properties of the organism were the result of protein functions.
- Proteins were assembled from free amino acids by translation
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
of an mRNA by ribosomes, tRNA and a group of related proteins.
- Ribosomes were composed of two subunits, one big and one small.
- Each ribosomal subunit was composed of a core of ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
surrounded by ribosomal proteins.
- The RNA molecules (rRNA and tRNA) played an important role in the catalytic activity of the ribosomes.
- Only 20 amino acids were used, to the exclusion of countless non-standard amino acids.
- Only the L-isomers
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
of the amino acids were used.
- Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
could be used as a source of energy and carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
; only the D-isomerIn chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
was used.
- ATP
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
was used as an energy intermediate.
- There were several hundred protein enzymes which catalyze chemical reactions that extract energy from fats, sugars, and amino acids, and that synthesize fats, sugars, amino acids, and nucleic acid bases using arbitrary chemical pathways.
- The cell contained a water-based cytoplasm that was surrounded and effectively enclosed by a lipid bilayer membrane.
- Inside the cell, the concentration of sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
was lower, and potassiumPotassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
was higher, than outside. This gradient was maintained by specific ion pumps.
- The cell multiplied by duplicating all its contents followed by cellular division.
Hypotheses
In 1859,
Charles DarwinCharles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
published
The Origin of Species in which he twice stated the hypothesis that there was only one progenitor for all life forms. In the summation at the end he says, "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed." (p 484). The very last sentence is a restatement of the hypothesis: "There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one." (p 490)
When LUA was hypothesized, cladograms based on
genetic distanceGenetic distance refers to the genetic divergence between species or between populations within a species. It is measured by a variety of parameters. Smaller genetic distances indicate a close genetic relationship whereas large genetic distances indicate a more distant genetic relationship...
between living cells indicated that
ArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
split early from the rest of life. This was inferred from the fact that all known archaeans were highly resistant to environmental extremes such as high salinity, temperature or acidity, and led some scientists to suggest that LUA evolved in areas like the deep ocean vents, where such extremes prevail today. But archaeans were discovered in less hostile environments and are now believed by many taxonomists to be more closely related to eukaryotes than
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, though this is still somewhat contentious.
In 1998,
Carl WoeseCarl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...
proposed (1) that no individual organism can be considered a LUA, and (2) that the genetic heritage of all modern organisms derived through
horizontal gene transferHorizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
among an ancient community of organisms. In 2010, based on "the vast array of molecular sequences now available from all domains of life", a formal demonstration of the validity of the Woese hypothesis was published.
The hypothesis is correct in its horizontal gene transfer prediction and incorrect in its community of organisms prediction. It is almost certain that (1) all of LUA's contemporaries became
extinctIn biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
and only LUA's genetic heritage lived to this day and (2) horizontal gene transfer took place.
Location of the root
The most commonly accepted location of the root of the tree of life is between a monophyletic domain Bacteria and a clade formed by Archaea and Eukaryota of what is referred to as the "traditional tree of life" based on several molecular studies starting with C. Woese. A very small minority of studies have concluded differently, namely that the root is in the Domain Bacteria, either in the phylum Firmicutes or that the phylum
ChloroflexiThe Chloroflexi or Chlorobacteria are a phylum of bacteria containing isolates with a diversity of phenotypes including members that are aerobic thermophiles, which use oxygen and grow well in high temperatures, anoxygenic phototrophs, which use light for photosynthesis, and anaerobic...
is basal to a clade with Archaea+Eukaryotes and the rest of Bacteria as proposed by
Thomas Cavalier-SmithProfessor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...
.
See also
- Timeline of evolution
This timeline of evolution of life outlines the major events in the development of life on planet Earth since it first originated until the present day. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations...
- Common descent
In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
- Abiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...
- Cell (biology)#Origin of the first cell
- Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
- Bacterial phyla
The bacterial phyla are the major lineages of the domain Bacteria.In the scientific classification established by Carl von Linné, each bacterial strain has to be assigned to a species , which is a lower level of a hierarchy of ranks...